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Animals are already dissolving in Southern Ocean

IN A small patch of the Southern Ocean, the shells of sea snails are dissolving. The finding is the first evidence that marine life is already suffering as a result of man-made ocean acidification.

Geraint Tarling of the British Antarctic Survey in Cambridge, and colleagues captured free-swimming sea snails called pteropods from the Southern Ocean in early 2008 and found that the outer layers of their hard shells bore signs of unusual corrosion (Nature Geoscience, doi.org/jtv). They say the region they sampled has a naturally low saturation of aragonite, a mineral that marine organisms use to build shells, but not so low that it would normally be a problem. But climate change has made the region an ocean-acidification danger zone.

As well as warming the planet, the carbon dioxide we emit is changing the chemistry of the ocean. CO2 dissolves in water to form carbonic acid, making the water less alkaline. The pH is currently dropping at about 0.1 per century, faster than any time in the past 300 million years.

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Lab studies show that organisms with hard shells will suffer as a result. To build their shells, they need to take up calcium carbonate from the water, but more carbonic acid due to emissions means more hydrogen ions in the water. These react with carbonate ions, which are then unavailable to form calcium carbonate.

The most vulnerable animals are those, like pteropods, that build their shells entirely from aragonite, a form of calcium carbonate that is very sensitive to pH. By 2050, much of the ocean will be undersaturated with respect to aragonite.

Aragonite is still relatively available in most of the ocean, but Tarling’s study shows the situation is already changing in some regions. The polar oceans will change fastest, he says, with the tropics following a few decades after.